HOW  OLD  ARE  FOSSILS? 


BY 

SHARAT  K.  ROY 
Assistant  Curator  of  Invertebrate  Paleontology 


Geology 
Leaflet  9 


FIELD  MUSEUM  OF  NATURAL  HISTORY 

CHICAGO 
1927 


LIST  OF  GEOLOGICAL  LEAFLETS  ISSUED  TO  DATE 

No.  1.  Model  of  an  Arizona  Gold  Mine $  .10 

No.  2.  Models  of  Blast  Furnaces  for  Smelting  Iron     .        .10 

No.  3.  Amber — Its  Physical  Properties  and 

Geological  Occurrence 10 

No.  4.  Meteorites 10 

No.  5.  Soils 10 

No.  6.  The  Moon 10 

No.  7.  Early  Geological  History  of  Chicago 25 

No.  8.  Agate 50 

No.  9.  How  old  are  Fossils  ? 25 

D.  C.  DAVIES,  Director 


FIELD  MUSEUM  OF  NATURAL  HISTORY 
CHICAGO,  U.  S.  A. 


LEAFLET  9. 


PLATE  I. 


A  VIEW  OF  THE  GRAND  CANYON  OF  THE  COLORADO  RIVER.  ARIZONA. 
ZOROASTER  IN  BACKGROUND. 
Photograph  by  O.  C.  Farrington. 


Field  Museum  of  Natural  History 

DEPARTMENT  OF  GEOLOGY 
Chicago,  1927 

Leaflet  Number  9 

How  old  are  Fossils? 

"How  old  is  that  fossil  and  how  do  you  know  it?"  is  a 
question  frequently  asked  by  visitors  going  through  the 
hall  of  fossils  in  the  Museum.  A  precise  answer  to  such  a 
question  is  impossible  and  an  adequate  one  demands  a 
longer  time  than  can  usually  be  afforded.  The  conse- 
quences of  inadequate  explanation  have  often  proved  to 
be  unsatisfactory.  The  visitor  becomes  skeptical  and 
instead  of  taking  interest  in  the  subject,  he  seems  to  be 
confirmed  in  his  doubts. 

In  this  leaflet  is  given  a  condensed,  general  statement 
of  methods  of  determining  the  age  of  ancient  life.  The 
information  is  drawn  from  the  works  of  various  authors, 
especially  Barrell's  "Rhythms  and  Measurements  of 
Geologic  Time."  It  is  intended  for  those  who  are  inter- 
ested in  the  age  of  past  life  and  yet  do  not  intend  an  ex- 
haustive study  of  the  subject,  nor  have  free  and  easy 
access  to  its  literature. 

The  birth  of  life  was  the  most  momentous  occasion 
in  the  history  of  the  earth.  When  one  considers  the  my- 
riads of  evidences  unearthed  by  paleontologists  and  paleo- 
botanists,  they  seem  to  leave  no  room  to  doubt  the  great 
conception  that  the  life  of  the  land  has  emerged  from  the 
sea.  It  is,  therefore,  only  a  natural  impulse  to  look  for  the 
remains  of  this  life  in  the  rocks  laid  down  by  the  ancient 
seas  and  to  wonder  at  the  vastness  of  time  behind  them. 

Since  traces  of  the  lowest  forms  of  life  have  been 
found  in  practically  the  oldest  known  sedimentary  strata, 

[141] 


2  Field  Museum  of  Natural  History 

the  problem  of  determining  the  age  of  life  necessarily 
involves  the  determination  of  the  age  of  those  strata.  But 
unravelling  the  dead  past  is  not  an  easy  task.  One  trying 
to  unlock  the  "secrets  of  the  cemetery  of  Nature's  dead," 
walks  on  a  shadowy  road.  His  difficulties  are  many. 
It  is  like  crossing  a  deep  moat,  climbing  a  steep  wall. 

Various  methods  have  been  applied  to  estimate  the 
age  of  different  periods  of  the  earth's  history  and  much  pro- 
gress has  been  made  toward  a  successful  issue.  Broadly, 
the  procedure  of  different  methods  is  the  same.  They  do 
not  differ  in  principle.  "The  rates  of  certain  changes  at  the 
present  day  are  determined  as  accurately  as  possible,  and 
in  imagination,  the  respective  processes  are  traced  back- 
ward in  time  until  limiting  conditions  are  arrived  at." 
Until  the  epoch-making  discovery  of  radium,  the  two  most 
outstanding  methods  used  in  calculating  geologic  time  were 
(1)  the  rate  of  land  erosion  and  deposition  and  (2)  the  rate 
of  derivation  of  salt  (sodium  chloride)  from  the  land  and 
its  accumulation  in  the  oceans.  Theoretically,  it  is  simple 
to  use  the  rate  at  which  sediments  are  being  deposited  or 
solutions  gathered  into  the  ocean,  as  "geologic  clocks"  for 
estimating  the  length  of  past  time.  But  in  practice  each 
method  encounters  its  own  difficulties  and  the  results  de- 
duced give  us  at  best  only  a  rough  idea  of  the  immensity  of 
time  involved.  I  shall  not  deal  with  these  individual 
estimates,  but  give  the  mean  of  several,  which  is 
100,000,000  years,  speaking  roundly.  As  stated  before,  it 
is  only  a  rough  estimate.  Nevertheless,  it  confirms  the 
fact  that  the  earth  is  very  old — indeed  much  older  than  is 
commonly  believed.  In  1650,  Bishop  Ussher,  in  his  inter- 
pretation of  the  "In  the  beginning"  of  Genesis,  estimated 
that  the  earth  was  created  4004  years  before  the  birth  of 
Christ.  According  to  this  view  the  earth  is  5931  years  old 
today.  Many  cosmogonists  and  even  some  geologists  of 
the  19th  century  held  this  Biblical  interpretation  to  be  the 
age  of  the  earth.  Other  ancient  religions  held  that  the 
earth  was  created  much  earlier  then  4004  B.  C.   Hutton, 

[142] 


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How  old  are  Fossils?  3 

one  of  the  founders  of  Geology,  in  his  studies,  found  "no 
vestige  of  a  beginning — no  prospect  of  an  end."  One  can- 
not help  sympathizing  with  Hutton.  Whoever  has  made  a 
trip  to  the  Grand  Canyon  of  the  Colorado  River,  Arizona, 
must  remember  the  awe-inspiring  depth  of  the  Paleozoic 
strata  and  thousands  of  feet  of  Proterozoic  sediments 
beneath  them  (PI.  I).  If  he  has  traveled  farther  north 
to  the  Cabinet  Range,  Montana,  he  must  have  carried 
with  him  an  undying  impression  of  the  35,000  ft.  of  the 
rocky  monument,  there  built  up  by  the  Proterozoic  seas. 

The  present  rate  of  denudation  in  the  Hudson  Bay  re- 
gion is  one  foot  in  about  47,000  years.  How  long  then,  has 
it  taken  the  seas  to  lay  down  these  miles  of  sediments,  which 
are  but  a  small  fraction  of  the  whole  geologic  column? 
Having  considered  all  this,  can  we  estimate,  even  approxi- 
mately, the  vast  length  of  time  that  has  elapsed  between 
these  periods  of  sedimentation,  the  so-called  "gaps"  or 
"breaks" — the  torn,  illegibly  written  pages  of  the  history 
of  the  earth?  Indeed,  there  are  moments  when  all  may 
feel  that  it  is  much  beyond  their  comprehension.  But, 
man,  by  nature,  is  at  once  humble  and  exalted.  He  is  will- 
ing to  admit  his  defeat,  yet  his  thirst  to  conquer  new 
knowledge,  to  know  the  truth,  is  never  satiated. 

The  reason  for  the  failure  to  arrive  at  an  absolute 
result  is  not  very  far  to  seek.  In  computing  geologic  time, 
one  has  to  calculate  that  which  has  elapsed,  by  some 
process  in  nature  that  takes  place  in  one  direction  only  and 
that  does  not  change  its  rate  when  conditions  alter.  What- 
ever the  method  applied,  be  it  the  deposition  of  sediments, 
or  the  gathering  of  solutions,  or  the  losing  of  heat  by  the 
sun  and  the  earth,  its  rate  of  action  should  be  uniform  and 
uninterrupted.  It  should  be  independent  of  the  changing 
conditions  of  the  earth.  Uniformity  of  the  rate  of  action 
is  the  criterion  for  precise  calculation.  But  we  know  that 
the  past  was  quite  different  from  the  present.  Different 
conditions  have  existed  at   different   times   during  the 

[143] 


4  Field  Museum  op  Natural  History 

earth's  history.  The  configuration  of  the  earth,  its  climate, 
humidity,  temperature  and  many  other  factors  have  varied 
from  time  to  time  and  with  them  the  rate  of  erosion, 
deposition  and  solution  has  either  accelerated,  diminished 
or  ceased.  The  doctrine  of  uniformitarianism  cannot  be 
assumed  in  a  changing  world,  even  though  our  knowledge 
of  the  earth  of  the  past  can  only  be  gained  from  a  fuller 
study  of  the  earth  of  the  present. 

When  we  consider  the  rate  of  sedimentation  as  a 
method  for  estimating  geologic  time,  we  take  the  total 
observed  thickness  of  the  geologic  column,  (estimated  to 
be  70  miles)  and  divide  it  by  the  rate  at  which  the  sediments 
are  now  being  laid  down.  But  do  we  know  this  rate?  By 
taking  the  average  rate  of  sedimentation  of  nine  large, 
rivers*  now  in  existence  and  assuming  it  to  be  the  rate  at 
which  sediments  were  deposited  in  the  past,  an  approxi- 
mate conclusion  can  be  arrived  at.  But  it  will  be  noticed 
from  the  figures  of  the  rate  of  sedimentation  of  different 
rivers  that  they  are  widely  variable,  the  highest  being 
many  times  greater  than  the  lowest.  Can  we  then  use  an 
average  of  nine  figures  so  out  of  proportion  and  yet  expect 
a  reliable  quantitative  value?  Furthermore,  the  rate  of 
deposition  along  the  coast,  near  the  mouth  of  a  large  and 
active  river,  is  much  higher  than  on  a  coastline  where  no 


River 

Drainage  areas 
in  square  miles 

Total  tons 
annually 

Ratio  of  sedi- 
ment to  water 
in  weight 

Height  in  feet  of 

column  of  sediment 

with  a  base  of  1 

square  mile 

Potomac 

Mississippi 

Rio  Grande 

Uruguay 

Rhone 

Po 

Danube 

Nile 

Irrawaddy 

11,043 

1,244,000 

30,000 

150,000 

34,800 

27,100 

320,300 

1,100,000 

125,000 

5,557,250 

406,250,000 

3,830,000 

14,782,500 

36,000,000 

67,000,000 

108,000,000 

54,000,000 

291,430,000 

1:3,575 
1:1,500 
1:    291 
1:10,000 
1:1,775 
1:    900 
1:2,800 
1:2,050 
1:1,610 

4.0 

241.4 

2.8 

10.6 

31.1 

59.0 

93.2 

38.8 

209.0 

Babb.  Science,  Vol.  XXI,  p.  343,  1893. 

[144] 


How  old  are  Fossils?  5 

rivers  empty  their  sediments  into  the  sea.  The  rate  is  also 
largely  controlled  by  the  character  of  the  sediments  de- 
posited. Sandstone  and  shale  are  more  rapidly  deposited 
than  limestone.  Moreover,  just  as  there  is  no  knowledge  of 
the  duration  of  time  of  erosion  between  periods  of  sedimen- 
tation, there  is  also  no  record  of  the  amount  of  detritus 
that  has  fallen  off  the  edge  of  the  continental  shelf  during 
widespread  emergence  of  the  continents. 

We  have  also  no  record  of  the  vast  quantity  of 
shallow  water  sediments  that  were  stirred  up  by  the 
penetration  of  storm  waves,  and  carried  to  abyssal 
depths  by  the  currents  and  tides. 

Similar  uncertainties  beset  us  when  we  consider  the 
rate  of  chemical  denudation,  that  is,  the  rate  at  which 
salts  have  been  dissolved  from  the  lands  and  accumulated 
in  the  oceans,  as  a  measure  of  geological  time.  Here, 
again,  we  take  the  total  amount  of  salts  that  is  in 
the  oceans  today  and  divide  it  by  the  present  rate  of  an- 
nual supply.  We  know  with  reasonable  accuracy  the 
quantity  of  salts  in  the  oceans  and  if  it  were  possible  to 
assume  the  present  rate  of  supply  to  be  a  true  mean  for  all 
geological  time,  a  satisfactory  age  of  the  oceans  might  be 
obtained.  But  it  cannot  be  assumed  as  such.  An  assump- 
tion of  this  nature  will  only  lead  us  from  the  domain  of 
exactness  to  that  of  uncertainty.  Aside  from  various 
other  factors,  neither  the  area  of  the  continents,  nor  their 
relief  was  in  the  past  the  same  as  today.  Consequently, 
the  stream  gradient  and  its  power  of  dissolving  salts  from 
the  land  surfaces  have  not  been  the  same.  It  is  also  not 
known  how  much  salt  the  ocean  derived  from  the  shore 
line  and  from  beds  beneath  the  ocean,  nor  how  much  of 
the  rock-salt  beds  on  the  earth  that  has  been  precipitated 
out  of  ocean  water. 

It  is  plain,  therefore,  that  the  rate  of  any  process  that 
is  controlled  by  so  many  conditions  cannot  be  used  (even 
making  generous  allowances  for  irregularities  and  inaccess- 
ible data)  as  a  reliable  guide  to  evaluate  geologic  time.   "It 

[145] 


6  Field  Museum  of  Natural  History 

is  a  clock,"  says  Harker,  "which  now  hurries  and  now 
creeps  or  stands  still,  and  it  cannot  be  trusted  as  a  time- 
keeper." 

Any  estimate  based  on  the  temperature  of  the  earth, 
or  of  the  sun,  encounters  similar  practical  difficulties,  for 
the  temperature  of  a  body  may  not  be  constant.  It  may 
rise  or  it  may  fall.  Further,  the  rate  of  change  of  temper- 
ature is  controlled  by  a  variety  of  conditions,  such  as  the 
amount  of  energy  radiated,  the  supply  of  energy  and  so 
forth.  Nor  is  there  any  record  of  the  immense  quantity  of 
heat  produced  by  igneous  agencies  and  radio-activity. 

Another  estimate,  one  of  the  earliest,  was  based  on 
the  rate  of  life  transformation  in  successive  periods.  The 
geological  series  were  divided  into  twelve  periods  and  it 
was  believed  that  20,000,000  years  were  required  for  an 
entire  change  in  the  species  of  each  period,  or  240,000,000 
years  in  all.  This  does  not  include  the  time  in  which  we 
have  no  record  of  plant  or  animal  life. 

There  is  no  reasonable  debate  as  to  the  passage  of  one 
species  to  another.  It  is  clearly  manifested  in  the  success- 
ion of  fauna  found  today  all  over  the  world  in  the  sedi- 
mentary rocks.  Even  the  most  casual  student  of  paleon- 
tology is  convinced  of  this  glaring  truth.  "The  brutal 
cogency  of  a  slab  of  fossils  could  be  hated  and  fought,  but 
could  not  be  gainsaid."  But  when  we  are  confronted  with 
the  question  of  setting  a  standard  of  measuring  geologic 
time  by  means  of  this  paleontological  record,  more  precis- 
ely, through  this  biological  process,  we  cannot  help  ponder- 
ing over  the  grave  uncertainty  of  the  result.  When  we  fix 
our  gaze  upon  a  trilobite,  a  three-lobed,  crab-like  creature 
(PI.  II  fig.  1)  that  ruled  the  seas  in  the  dim  days  of  the  Cam- 
brian period  (p.  11)  and  see  that  it  was  equipped  with  gills 
and  swimming  organs,  with  powers  of  digestion  and  ex- 
cretion, with  specific  organs  of  circulation  and  reproduction 
and  with  motor  and  sensory  nerves,  and  compare  it  with  one 
of  its  tribe,  a  present  day  horseshoe  crab,  (PI.  II  fig.  2)  we  do 

[146] 


How  old  are  Fossils?  7 

not  find  any  noticeable  progress  in  structure,  in  intricacy 
or  in  the  degree  of  specialization.  Yet  the  time  that  has 
elapsed  since  the  Cambrian  is,  according  to  a  moderate 
estimate,  nearly  600,000,000  years!  (p.  11) .  Geologic  record 
testifies  that  evolution  awaits  environmental  change,  that 
animals  in  some  way  adjust  themselves  to  their  environ- 
ment, either  by  discarding  or  modifying  old  characters  or  by 
acquiring  new  ones.  Yet,  what  are  known  as  "immortal" 
types,  such  as  the  brachiopods,  Lingula,  Crania  and  Tere- 
bratula  (PI.  Ill  figs.  1-3)  or  the  pelecypods,  such  as  Pecten, 
Pinna  and  Area  (PI.  Ill  figs.  4-6)  or  the  gastropods,  such  as 
Pleurotomaria,  Natica  and  Trochus  (PI.  Ill  figs.  7-9),  have 
withstood  all  possible  environmental  changes  and  have 
steadfastly  held  their  own  ever  since  we  have  records  of  their 
very  early  appearance  on  earth.  On  the  contrary,  we  have 
records  of  types  that  have  yielded  so  rapidly  to  change  that 
their  evolution  is  almost  explosive.  It  is  almost  incompre- 
hensible how,  within  such  a  limited  period  of  time,  fishes 
have  changed  into  amphibians,  amphibians  into  reptiles, 
and  reptiles  into  birds  and  mammals  (PI.  IV  figs.  1-4). 
With  these  conflicting  evidences  staring  us  in  the  face, 
with  the  knowledge  that  the  entire  organic  world  has  been 
subject  to  earth-wide  periods  of  long  stagnation  and  rapid 
intensive  change,  one  may  well  ponder  whether  it  is  within 
our  power  to  establish  a  standard  for  measuring  geologic 
time  on  the  evidence  of  life  transformation.  The  study  of 
the  succession  of  faunas — the  change  of  one  species  to 
another,  can  only  indicate  the  magnitude  of  time  in- 
volved. It  cannot  afford  any  basis,  whatsoever,  for  a 
concrete  expression  of  geologic  time. 

During  the  last  three  decades,  a  number  of  radio- 
active changes  of  one  chemical  element  into  another  have 
been  discovered  and  studies  of  certain  minerals  and  rocks 
containing  various  radio-active  elements  have  created 
means  to  calculate  their  age  with  remarkable  accuracy. 
"A  study  of  the  various  radio-active  elements  contained  in 

E147] 


8  Field  Museum  of  Natural  History 

minerals  and  rocks,"  says  Harker,  "has  shown  that  it  is 
possible,  in  certain  favorable  cases,  to  calculate  directly 
their  age  in  years." 

The  radio-active  minerals  are  commonly  found  in 
igneous  rocks.  They  are  widely  distributed  all  over  the 
world.  The  parents  of  the  whole  series  of  radio-active 
elements  are  uranium  and  thorium.  They  possess  the 
highest  atomic  weights  of  all  known  elements.  Each  of 
these  parental  elements  transforms  itself  through  a  success- 
ion of  changes.  The  final  product  of  uranium  is  the 
formation  of  the  metal  lead  and  the  gas  helium.  These 
transformations  take  place  in  one  direction  only,  that  is, 
from  an  element  of  higher  atomic  weight  to  an  element  of 
lower  atomic  weight.  It  has  also  been  demonstrated 
beyond  question  that  these  transformations  are  unalterable 
by  any  process  whatsoever  and  that  they  are  independent 
of  temperature,  pressure  or  any  other  physical  or  chemical 
state.  Temperatures  up  to  2,500  C.  and  pressures  up  to 
600  tons  per  square  inch  have  not  been  found  to  influence 
the  rate  of  transformation.  Time  estimated  on  the  basis 
of  these  processes,  therefore,  offers  a  more  reliable  result 
than  that  obtained  by  any  other  method  hitherto  known. 
Detailed  descriptions  of  how  the  metal  uranium  slowly 
and  regularly  breaks  down  in  a  descending  series  into  the 
metal  lead  and  the  gas  helium,  will  be  found  in  the  litera- 
ture on  radio-activity.  For  our  purpose,  it  suffices  to  say 
that  according,  to  Barrell,  an  atom  of  uranium  which 
breaks  up  will  ultimately  give  rise  as  a  stable  product  to 
eight  atoms  of  helium  and  one  atom  of  lead.  Since  the 
rate  of  transformation  is  known,  data  for  calculating  the 
age  of  the  mineral  and  with  it  the  rock  formation  of  which 
it  is  a  part,  can  be  obtained  by  measuring  the  quantity  of 
helium  and  lead  in  the  rock  and  comparing  it  with  the 
quantity  of  uranium  in  the  same  volume  of  material.  But, 
as  helium  is  a  gas,  it  is  likely  that  a  certain  portion  of  it 
leaks  out  and  consequently  the  estimate  of  age  on  the 
basis  of  how  long  helium  had  been  in  contact  with  uranium 

[148] 


How  old  are  Fossils?  9 

and  lead  is  to  be  regarded  as  a  minimum  estimate.  For 
example,  the  age  of  the  mineral  thorianite  that  occurs 
abundantly  in  the  sands  and  gravels  of  Ceylon  has  been 
estimated  to  be  280,000,000  years,  but  the  mineral  is 
doubtless  much  older,  as,  ever  since  it  was  broken  away 
from  its  orginal  home  in  the  pegmatite  dikes  of  Ceylon,  it 
lay  exposed  to  the  action  of  weathering  and  it  was,  there- 
fore, very  likely  that  during  all  these  years  a  certain  per- 
centage of  its  helium  contents  had  leaked  away. 

But  estimates  based  on  the  lead  ratios  of  radio-active 
minerals  offer  results  consistent  among  themselves.  That 
is,  whenever  fresh,  primary,  uranium-bearing  minerals  of 
the  same  geological  age  have  been  examined,  the  lead  ratios 
are  always  found  to  remain  constant.  The  value  of  the 
ratios  increases  or  decreases  as  the  geological  age  of  the 
respective  mineral  increases  or  decreases.  In  other  words, 
the  lead  ratios  are  in  keeping  with  the  geological  age. 

The  procedure  of  applying  the  lead  ratio  in  calcu- 
lating geological  time  can  be  briefly  stated  thus:  The  rate 
of  production  of  lead  from  uranium  can  be  readily  calcu- 
lated. The  rate  at  which  helium  is  generated  is  accurately 
known  and  the  quantity  of  lead  liberated  in  the  same  time 
is  approximately  6.5  times  that  of  helium.  In  a  year  one 
gram  of  uranium  produces  1.25  x  10"10  grams  of  lead,  and 
at  this  rate  8,000*  million  years  will  be  required  for  the 
production  of  one  gram  of  lead. 

There  is  no  serious  difficulty  in  applying  this  method 
for  measuring  geologic  time,  except  that  it  is  necessary  to 
determine  whether  the  lead  is  of  radio-active  origin  or 
original  lead.  The  presence  of  original  lead  is  likely  to  mar 
the  constancy  of  the  lead  ratio  essential  for  accurate 
results.  But  ordinary  lead  need  not  be  confused  with 
uranium  lead,  as  the  atomic  weight  of  ordinary  lead  is 
207.1  and  that  of  uranium  lead  is  206.2.  Values  between 

*A  more  recent  and  accurate  computation  reduces  this  8,000 
million  years  to  7,500  million  years. 

[149] 


10  Field  Museum  of  Natural  History 

these  two  figures  imply  a  mixture  of  two  types  of  lead. 
For  reliable  calculations,  a  series  of  fresh,  primary  minerals 
of  the  same  geological  age  showing  a  constant  lead  ratio  of 
atomic  weight  206.2  needs  to  be  examined. 

The  following  table  shows  the  geologic  time  that  has 
elapsed  between  the  first  evidences  of  life  and  the  present, 
as  calculated  by  Barrell  from  radio-active  data.  The  fig- 
ures are  his  minimum  and  maximum  estimates.  It  will  be 
noticed  from  the  figures  in  the  table  that  the  earliest  life  of 
which  we  have  fossil  records  is  about  1,500,000,000  years 
old.  From  this,  it  could  be  safely  concluded  that  the  in- 
ception of  life  on  earth  must  have  taken  place  much 
earlier.  It  is  quite  significant  that  each  geological  era, 
occasionally  a  geological  period,  has  its  characteristic 
grouping  of  life  developed  from  the  life  of  preceding 
periods.  As  we  climb  higher  in  the  geological  column,  life 
becomes  more  and  more  complex  and  specialized.  From 
the  one-celled  life  of  the  Archeozoic  it  passes  through  the 
invertebrates — fishes — amphibians — reptiles — birds  and 
mammals  to  man  of  the  Recent  time. 

Since  Barrell's  publication  of  the  estimates  of  geologic 
time  as  measured  by  means  of  radio-activity,  some  further 
studies  have  been  made  along  the  same  line,  but,  as  no 
generally  accepted  results  have  shown  any  marked  differ- 
ences from  Barrell's  results,  it  has  been  deemed  advisable 
to  use  his  age  data  as  perhaps  our  present  most  adequate 
guide  as  to  the  length  of  geologic  periods. 
%\  p  Although  the  measurable  forces  of  radio-activity  give 
on  the  whole  a  remarkably  satisfactory  time  gauge  and  are 
doubtless  more  accurate  than  any  method  here  discussed, 
it  must  not  be  considered  that  the  ages  given  (p.  11)  are 
absolute.  The  knowledge  of  geological  time  is  of  more  im- 
portance for  the  comparative  than  for  the  absolute  magni- 
tude of  the  results  obtained .  Just  as  the  study  of  astronomy 
gives  us  the  conception  of  the  vastness  of  space,  so  does  the 
study  of  geology  reveal  to  us  that  of  the  immensity  of  time. 

Sharat  K.  Roy. 

[150] 


LEAFLET  9. 


v* 


"IMMORTAL"  TYPES. 

1.    LINGULA.      2,   CRANIA.      3.   TEREBRATULA.     4,   PECTEN.      5,    PINNA.      6,   ARCA. 

7,    PLEUROTOMARIA.      8,   NATICA.      9,   TROCHUS. 

(Figs.  I,  2,  7,  after  Hall,  3,  9,  after  Zittel,  4,  6,  after  Dall,  5,  Pal.  N.  J.  I,  8,  after  Cragin). 

Drawings  by  Carl  F.  Gronemann. 


THE  GEOLOGICAL  TIME  TABLE 

1.0  =  One  million  years 


11 


ERAS 

PERIODS 

TIME  SCALE 
(After  Barrell) 

characteristic 

Minimum 

Maximum 

PSYCHOZOIC 

Recent 

1 

1.5 

Age  of  Man 

Pleistocene 

Pliocene 

7 

9 

CENOZOIC 

Miocene        # 

19 

23 

Age  of  Mammals 

and  Modern 
Flowering  Plants 

Oligocene 

35 

39 

Eocene 

55 

65 

Cretaceous 

95 

115 

MESOZOIC 

Comanchian 

120 

150 

Age  of  Reptiles 

Jurassic 

155 

195 

Triassic 

190 

240 

Permian 

215 

280 

Age  of  Amphibians 

AND 

Pennsylvanian 

250 

330 

Ancient  Floras 

Mississippian 

300 

370 

Age  of  Fishes 

PALEOZOIC 

Devonian 

350 

420 

Silurian 

390 

460 

Age  of  Higher 
Shelled 

Ordovician 

480 

690 

Cambrian 

550 

700 

PROTEROZOIC 

Systematic 

classification 

variable 

925 

Age  of  Primitive 
Invertebrates 

Long  Erosional  Interval 

ARCHEOZOIC 

1500 

Dawn  of  Unicel- 
lular Life,  Algal 
Forms  Reported 

BIBLIOGRAPHY 

Barrell,  J. — Rhythms  and  Measurements  of  Geological  Time, 
Bull.  Geol.  Soc.  Am.,  Vol.  28,  1917,  pp.  745-904. 

Becker,  Geo.  E, — The  Age  of  the  Earth.  Smithsonian! Misc. 
Coll.,  LVL,  No.  6,  1910. 

Chamberlain,  T.  C. — Diastrophism  and  the  Formative  Processes, 
XIII.  The  Time  over  which  the  Ingathering  of  the  Planetesi- 
mals  was  Spread,  Jour.  Geol.,  Vol.  XXVIII,  1920,  pp.  675-81. 

Harker,  A. — Geology  in  relation  to  the  exact  sciences,  with  an 
excursus  on  geological  time.  Nature,  Vol.  95,  1915,  pp.  105- 
109. 

Holmes,  A. — The  Age  of  the  Earth.    Harper  and  Bros.,  London 
.     and  New  York,  1913. 

Joly,  J.— Radioactivity  and  Geology,  Van  Nostrand,  N.  Y.,  1909; 
An  Estimate  of  the  Geological  Age  of  the  Earth.  Trans.  Roy. 
Soc.  Dublin,  VII.  1899,  pp.  23-66;  The  Age  of  the  Earth, 
Phil.  Mag.,  6th  ser.,  Vol.  XXII,  1911,  pp.  359-80. 

Sollas,  W.  J. — Presidential  Address.  Quart.  Journ.  Geol.gSoc, 
Vol.  65,  1909,  pp.  cxii;  Proc.  Geol.  Soc.  of  London, ^Sess. 
1908-9,  pp.  i-cxxii. 

Walcott,  C.  D. — Geologic  Time,  as  indicated  by  the  sedimentary 
rocks  of  North  America.  Smithsonian  Rep.  1893,  pp.  301-334; 
Jour.  Geol.  Vol.  Ill,  1893,  pp.  639-674. 


LEAFLET  9. 


m 


EVOLUTION  OF  FISH  TO  MAMMAL-LIKE  REPTILE. 

1,    FISH.      2,   AMPHIBIAN.      3,    REPTILE.     4.    MAMMAL-LIKE  REPTILE. 

(Figs.  1,  2,  after  Klaatsch,  3,  based  on  Williston,  4,  based  on  Gregory). 

Drawings  by  Sharat  K.  Roy. 


PRINTED  IN  THE  UNITED  STATES  OF  AMERICA 
BY  FIELD  MUSEUM  PRESS 


